1. Field of the Invention
The present invention relates to a communications network, and in particular to a method and apparatus for preventing overloads in such a network.
2. Related Art
Some of the most demanding traffic patterns on a telecommunications network relate to the operation of telemarketing call answering centres. For example, the number of a call answering centre might be displayed as part of a nationally broadcast advertisement for a product. This may result in many thousands of calls being made to the number in a short period of time following the advertisement. Such call levels have the potential to overload the network at a number of points, including intermediate DMSU's (digital main switching units) and the destination exchange.
In networks which employ an IN (intelligent network) architecture, it has previously been proposed to equip IN platforms with a call gapping mechanism. This enables the platform to transmit an instruction to a service switching point (SSP) to reduce the rate of inbound calls to the IN platform from the SSP. However, although this mechanism is effective to prevent overloading at the IN platform, it does not in itself provide adequate protection for a destination exchange which is downstream from the platform. The IN platform has a call handling capacity which is typically several times greater than the capacity of an individual exchange. Therefore, long before the call gapping mechanism is invoked to protect the IN platform, the platform may be passing calls to a destination exchange at a rate which is sufficient to bring a risk of software failure at the destination exchange. Even if the destination exchange withstands a sudden peak in traffic and passes the calls on to the answering centre, if the capacity of the answering centre is exceeded then many of the calls will not be completed but will result in a BUSY signal being returned to the caller. Ineffectives , as such calls are known, earn no revenue for the network operator, nor for the answering centre, but add to the network traffic and so have to be supported by the network infrastructure.
In order to prevent overloading of the destination exchange, and to reduce the number of ineffectives, the present applicant has previously proposed mechanisms for controlling call rates on the outward leg from a network platform to a destination exchange. International Patent Application no. PCT/GB 94/02512 discloses and claims an approach in which call levels are monitored and controlled to maintain a predetermined and relatively low level of BUSY signals from the call answering centre. This has helped to ensure that the resources of the answering centre are used effectively while protecting the local exchange from overload. However while it reduces to an extent the number of ineffectives on the network, a significant number remain. Furthermore, since a predetermined number of excess calls are admitted for each answering centre, the total number of ineffectives rises when call answering is distributed over several centres.